1. Field of the Invention
The present invention relates to cardiac pacing systems and particularly to catheter apparatus usable in such cardiac pacing systems.
2. Description of the Prior Art
Bipolar pacing systems and particularly bipolar demand cardiac pacing systems in which a common pair of electrodes is employed for providing both stimulation and sensing of ventricular depolarization are well known. For example, such a bipolar demand cardiac pacing system is disclosed in U.S. Pat. Nos. 3,595,242; 3,478,746; 3,735,766, and my previous U.S. Pat. No. 3,908,667. Moreover, catheters for employment with such prior art cardiac pacing systems in which a predetermined spacing of the electrodes positioned in the heart is established by the catheter are also well known as disclosed, by way of example, in U.S. Pat. No. 3,915,174. However, with respect to the catheter disclosed in U.S. Pat No. 3,915,174, the resultant current flow may unintentionally involve the superior conduction system of the heart, in which instance, the cardiac pacer can induce undesirable ventricular arrhythmias, especially if the anode produces current through the super ventricular conduction system. The bipolar catheter disclosed in U.S. Pat. No. 3,915,174 recognizes this problem, however, it does not avoid stimulation of the super ventricular conductive system but, rather, merely makes an effort to minimize such stimulation by employing a large electrode in order to obtain a low current density. Thus, the problem may still occur.
As set forth in my previous U.S. Pat. No. 3,908,667, there are several disadvantages to such bipolar demand cardiac pacing which can be overcome by providing a tripolar demand cardiac pacing system, such as the system shown in FIG. 7 of that patent. Thus, in a tripolar demand cardiac pacing system, stimulation is applied between a pair of intra-ventricular electrodes whereas sensing of ventricular depolarization is obtained between an intra-ventricular electrode and an extra-ventricular electrode. The advantage of sensing between an intra-ventricular and an extra-ventricular electrode is that this pair of electrodes spans the entire ventricle and, therefore, receives the maximum ventricular depolarization signal, whereas the conventional bipolar arrangement of sensing between two closely adjacent electrodes located within the ventricle may result in failure to detect ventricular depolarization because the geometry of the depolarization wave front under certain conditions may not produce a sufficient voltage difference between such adjacent electrodes. Such a failure to sense a spontaneous depolarization would result in the demand cardiac pacer's reversion to the free running mode, with the danger of an electronic stimulus applied during this vulnerable period causing fibrillation. Even if such fibrillation does not occur, competition between the spontaneous cardiac cycle and the electronic cardiac stimulus may cause the aforementioned harmful arrhythmias. As was previously mentioned, stimulation applied between the extra-ventricular and intra-ventricular electrodes can present some danger in that the current through the extra-ventricular electrode may stimulate the upper conduction system consisting of the sino-atrial node, the atrio-ventricular node, and the conduction pathways between them. Such simultaneous stimulation of both the extra-ventricular and ventricular conduction systems departs substantially from the natural sequence of the heart and can cause undesirable results.
The tripolar arrangement described with reference to FIG. 7 of my aforementioned previous U.S. Pat. No. 3,908,667 overcomes several of these problems inherent in a bipolar cardiac demand pacing system. However, such a system as disclosed therein involves circuit redesign. Moreover, in certain instances, the physician may still wish to use bipolar cardiac demand pacing initially with subsequent conversion to tripolar cardiac demand pacing if certain problems arise. Applicant is presently unaware of any available prior art cardiac pacing systems which enable a conventional bipolar demand cardiac pacer to be employed as a tripolar demand cardiac pacer when the advantages of such tripolar demand cardiac pacing are desired. These disadvantages of the prior art are overcome by the present invention.